Method of treating wastewater

ABSTRACT

This disclosure describes wastewater treatment systems and methods of treating wastewater. In one exemplary method, a wastewater is split into first and second wastewater fractions. The first fraction is delivered to a membrane bioreactor, which may produce an effluent with a low pollutant concentration, and the second fraction is delivered to a biological wastewater treatment system, which may yield a higher pollution concentration yet have a shorter solids retention time. Some implementations of the invention can routinely meet or even exceed pollution discharge standards quite economically during normal operation, yet retain significant flexibility for handling seasonal or sudden variations in the flow rate of wastewater into the system. In select adaptations, waste activated sludge containing heterotrophs, autotrophs, and (optionally) polyphosphate accumulating organisms is delivered from the membrane bioreactor to the biological wastewater treatment system.

TECHNICAL FIELD

The present invention generally relates to wastewater treatment systemsand methods. Aspects of the invention enable production of ahigh-quality effluent from a wastewater influent at a reasonable costover a wide range of influent flow and pollutant loading rates.

BACKGROUND

Regulation of pollutant discharges from municipal wastewater treatmentsystems has become more stringent in recent years. In response, manymunicipalities have deployed new wastewater treatment systems orretrofitted existing systems to reduce pollutant discharge. Pollutantscan be of many forms with the most common being Biochemical OxygenDemand (BOD), Chemical Oxygen Demand (COD), Total Suspended Solids(TSS), ammonia, total nitrogen, nitrate, nitrite and phosphorous.

Biological treatment systems such as conventional activated sludgesystems and membrane bioreactors are one method to reduce the pollutantsin a wastewater influent. Biological treatment systems are designed andoperated to retain an adequate amount of activated sludge such that thepollutant load contained in the water treated by the system will beadequately reduced. The amount of activated sludge is related to theSolids Retention Time (SRT) of the system, and the minimum SRT requiredto treat various pollutants under various conditions is generally wellknown. Conventional activated sludge systems retain activated sludge bythe use of settling or clarification devices and can maintain adequateSRTs to treat pollutants provided that both the flow and the activatedsludge concentration to the settling basins or clarifiers are withinreasonable limits, which depend upon the area of the settling basins orclarifiers and the characteristics of the activated sludge. Membranebioreactor systems retain the activated sludge by the use of membranefiltration equipment and can operate successfully at significantlyhigher activated sludge concentrations than typical for conventionalactivated sludge systems, but are more limited in their ability toprocess occasional high flow rates.

The flow rate and pollutant load of an influent treated by a municipalwastewater system, which may include industrial wastewater, residentialwastewater, and precipitation runoff, can vary significantly over time.In addition to normal diurnal variations, significant rain events cancause short-term spikes in wastewater influent flow rate and pollutantload. Several systems have been developed that can accommodatesignificantly varying flow rates and pollutant loads, including membranebioreactors in combination with flow equalization storage tanks,membrane bioreactors with additional membranes for the treatment of peakflows, conventional activated sludge systems with parallel chemicaltreatment systems, and conventional activated sludge systems withoversized tanks and clarifiers. Each of these systems for accommodatingsignificantly varying flow rates and pollutant loads significantlyincreases the operating costs and land area requirements for awastewater treatment plant, and those systems utilizing parallelchemical treatment systems do not produce as high a quality of effluent.

Varying flow rates and pollution loads are just two factors that must beconsidered when designing a wastewater treatment system. Thecharacteristics of the wastewater, including its temperature and thetypes of pollutants that it contains, are another. Many biologicaltreatment systems employ two types of biological material to reduce awastewater's ammonia, organic material, and nitrate concentrations:autotrophic organisms, which are also called “nitrifiers,” are used toconvert ammonia to nitrate and heterotrophic organisms are used toremove organic materials and nitrates. The growth rates of “nitrifiers”are usually much lower than those of the heterotrophic organisms.Moreover, the wastewater temperature can significantly impact the growthrate of nitrifiers. See, e.g., Grady, et al., Biological WastewaterTreatment, Second Edition, Marcel Dekker, N.Y. (1999).

In northern climates, winter wastewater temperatures are sometimes 10°C. or lower. To ensure sufficient nitrification to meet dischargerequirements during winter months, wastewater treatment systems in suchclimates are typically designed with solids retention times of 8 days orlonger. Longer solids retention times require systems with a largerfluid capacity. Increasing the solids retention time in conventionalbiological treatment systems also increases the mixed liquor suspendedsolids concentrations, requiring larger-capacity aeration systems andsecondary clarifiers, which tends to increase expenses for a wastewatertreatment plant because of factors such as higher permitting andoperating costs and greater land area requirements.

As an alternative to larger aeration systems and secondary clarifiers,some systems achieve good nitrogen removal at low solids retention timesby nitrifier bioaugmentation, in which nitrifiers are added from aseparate seed source. For example, wastewater treatment systems thatinclude certain trickling filters/activated sludge processes canpartially nitrify ammonia in the trickling filter. See, e.g., Daigger,et al., “Process and Kinetic Analysis of Nitrification in CoupledTrickling Filter/Activated Sludge Processes,” Water EnvironmentResearch, Vol. 65, pp. 679-685 (1993). Nitrifiers grown in the tricklingfilter are allowed to slough off the filter to “seed” the activatedsludge process, enabling stable nitrification at decreased solidsretention times in the suspended growth bioreactor.

Another example of nitrifier bioaugmentation is described by Constantinein a 1996 Masters thesis at McMasters University entitled“Bioaugmentation to Achieve Nitrification in Activated Sludge Systems.”Two parallel sequencing batch reactors are operated at two differentsolids retention times. One sequencing batch reactor is operated at asolids retention time long enough to ensure thorough nitrification; thisreactor is called the “donor” reactor. The other sequencing batchreactor is operated at a solids retention time too short to allowsignificant nitrification; this reactor is called the “receiver”reactor. Waste activated sludge is directed from the donor to thereceiver reactor, resulting in a constant supply of nitrifiers from thedonor to the receiver reactor. This allows significant nitrification tooccur in the receiver reactor at solids retention times less than thoserequired absent the bioaugmentation. (See also Constantine's U.S. Pat.No. 6,723,244.)

Tendaj-Xavier proposed a two-stage wastewater treatment process in a1983 dissertation at the Royal Technical University entitled “BiologicalTreatment of Sludge Water from Centrifugation of Digested Sludge.” Thedissertation generally suggests growing nitrification bacteria on thedewatering centrate produced within a wastewater plant and seeding thenitrified bacteria into the primary wastewater stream. However,Tendaj-Xavier observes that the high initial capital expense and/orincreased space requirements of the proposed system could beprohibitive.

The process described in U.S. Pat. No. 5,811,009 (Kos) also relies onbioaugmentation. See also P. Kos, “Short SRT (Solids Retention Time)Nitrification Process/Flowsheet,” Wat. Sci. Tech., Vol. 38, No. 1, pp.23-29 (1998). The Kos process configuration mitigates some drawbacks ofthe process proposed by Constantine (1996). Kos' system adds asidestream reactor, which is distinct from the main waste treatmentstream, to treat recycle streams from anaerobic digesters. These recyclestreams are rich in ammonia, which is released during anaerobicdigestion, and support an enriched culture of nitrifiers in thesidestream reactor. The temperature of the recycle streams is oftenelevated, which promotes nitrification. Kos' steady-state simulationssuggested that the described process allowed nitrification of the mainwaste treatment stream at reduced solids retention times.

The Kos process configuration has potential problems, especially withrespect to the sidestream plant operation. For example:

-   This system may require high supplementary alkalinity to maintain    process stability.-   Substrate and product inhibition in this process may render the    process unstable, as described in Anthonisen, et al., “Inhibition of    nitrification by ammonia and nitrous acid,” Journal of the Water    Pollution Control Federation, Vol. 48, pp. 835-852 (1976).-   The poor settling characteristics of enriched nitrifier cultures may    interfere with maintaining consistent solids retention times in the    side stream reactor. See, e.g., “U.S. Environmental Protection    Agency, Process Design Manual for Nitrogen Control,”    EPA/625/R-93/010, U.S. Environmental Protection Agency, Cincinnati,    Ohio (1993).

The entirety of each of these patents and other publications, and of anypatents or other publications referred to below, is incorporated hereinby reference.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic overview of a wastewater treatment system inaccordance with an embodiment of the invention.

FIG. 2 is a schematic overview, similar to FIG. 1, of a wastewatertreatment system in accordance with another embodiment of the invention.

DETAILED DESCRIPTION

A. Overview

Various embodiments of the present invention provide systems and methodsfor treating wastewater. Many embodiments of the invention are capableof receiving an influent that exceeds one or more environmentalstandards and discharging an effluent that meets current environmentalstandards, including limitations on BOD, COD, TSS, ammonia, nitrate,nitrite, total nitrogen, and phosphorus levels. Aspects of the inventionmay be selected to maximize treatment efficacy during “normal”operation, yet yield acceptable discharge quality with the same systemeven during high input periods.

A method of treating wastewater in accordance with one embodimentincludes splitting wastewater that has a concentration of a pollutantinto a first wastewater fraction having a first concentration of thepollutant that exceeds the permitted discharge standard for thepollutant and a second wastewater fraction having a second concentrationof the pollutant that exceeds the permitted discharge standard for thepollutant. The first and second concentrations of the pollutant may bethe same. The first wastewater fraction, which contains approximatelythe first concentration, may be delivered to a membrane bioreactor. Aneffluent is produced from the membrane bioreactor. This effluent has aconcentration of the pollutant less than the permitted dischargestandard for the pollutant. The second wastewater fraction, whichcontains approximately the second concentration, may be delivered to abiological wastewater treatment system. An effluent is produced from abiological wastewater treatment system. This effluent has aconcentration of the pollutant greater than the permitted dischargestandard. At least a portion of the effluent from the membranebioreactor is combined with at least a portion of the effluent from thebiological wastewater treatment system to create a blended effluenthaving a concentration of the pollutant no greater than the permitteddischarge standard.

An alternative embodiment of the invention provides a wastewatertreatment system that includes an influent splitter, a first wastewaterprocessor, a second wastewater processor, and an effluent path. Theinfluent splitter is configured to receive an influent, which has aconcentration of a pollutant that exceeds the permitted dischargestandard for the pollutant, and to deliver a first fraction of theinfluent to a first inlet and deliver a second fraction of the influentto a second inlet. The first wastewater processor is in fluidcommunication with the first inlet and includes a first outlet and amembrane bioreactor in a first wastewater path from the first inlet tothe first outlet. The first wastewater processor is configured toproduce a first effluent having a concentration of the pollutant lessthan the permitted discharge standard. The second wastewater processoris in fluid communication with the second inlet and includes a secondoutlet and a biological wastewater treatment system in a secondwastewater path from the second inlet to the second outlet. The secondwastewater processor is configured to produce a second effluent that hasa concentration of the pollutant greater than the permitted dischargestandard. The effluent path is configured to receive at least a portionof the first effluent and at least a portion of the second effluent anddeliver a blended effluent that has a concentration of the pollutant nogreater than the permitted discharge standard.

Another embodiment of the invention provides a method of treatingwastewater that includes splitting a wastewater flow, which has a loadfor a pollutant that exceeds a permitted discharge standard for thepollutant, into a first wastewater fraction having a first load for thepollutant and a second wastewater fraction having a second load for thepollutant. When the load is no greater than a load capacity of amembrane bioreactor (which has a first solids retention time), the firstload is at least about 35% of the load; when the load is greater thanthe load capacity of the membrane bioreactor, the first load is no lessthan about 50%, e.g., 75% or greater, of the load capacity of themembrane bioreactor. The first wastewater fraction, which containsapproximately the first load, is delivered to the membrane bioreactor.An effluent is produced from the membrane bioreactor; this effluent hasa load for the pollutant less than the permitted load discharge standardfor the pollutant. The second wastewater fraction, which containsapproximately the second load, is delivered to a biological wastewatertreatment system that has a second solids retention time. The secondsolids retention time is no greater than the first solids retention timeand is optimally substantially less than the first solids retentiontime.

A method of treating wastewater in accordance with still anotherembodiment of the invention includes splitting a wastewater flow, whichhas a flow rate and a concentration for a pollutant that exceeds apermitted discharge standard for the pollutant, into a first wastewaterfraction having a flow rate and a second wastewater fraction having aflow rate. When the flow rate of the wastewater flow is no greater thana flow rate capacity of a membrane bioreactor, which has a first solidsretention time, the flow rate of the first wastewater fraction is atleast about 50% of the flow rate; when the flow rate of the wastewaterflow is greater than the flow rate capacity of the membrane bioreactor,the flow rate of the first wastewater fraction is no less than about50%, e.g., 75% or greater, of the flow rate capacity of the membranebioreactor. The first wastewater fraction is delivered to the membranebioreactor and an effluent is produced from the membrane bioreactor.This effluent has a concentration for the pollutant that is less thanthe permitted discharge standard for the pollutant. The secondwastewater fraction is delivered to a biological wastewater treatmentsystem that has a second solids retention time. The second solidsretention time is no greater than, and is desirably substantially lessthan, the first solids retention time.

Another alternative embodiment provides a wastewater treatment systemthat includes a first wastewater processor, a second wastewaterprocessor, an influent splitter, and a programmable controller. Thefirst wastewater processor includes a first inlet, a first outlet, and amembrane bioreactor in a first wastewater path from the first inlet tothe first outlet. The membrane bioreactor has a first solids retentiontime and a flow rate capacity. The second wastewater processor includesa second inlet, a second outlet, and a biological wastewater treatmentsystem in a second wastewater path from the second inlet to the secondoutlet. The biological wastewater treatment system has a second solidsretention time that is shorter than the first solids retention time. Theinfluent splitter is configured to deliver a first fraction of aninfluent having a flow rate to the first inlet and to deliver a secondfraction of the influent to the second inlet. The programmablecontroller is operatively connected to the influent splitter to controlflow rate parameters of the first and second wastewater fractions suchthat: when the flow rate of the influent is no greater than the membranebioreactor flow rate capacity, the first flow rate is at least about 65%of a flow rate of the influent; and when the flow rate of the influentis greater than the bioreactor flow rate capacity, the first flow rateis no less than about 75% of the membrane bioreactor flow rate capacity.

Yet another embodiment provides a method of treating wastewater with asystem that includes a membrane bioreactor having a bioreactor targetflow rate and a biological wastewater treatment system having a minimumflow rate. A wastewater flow that has a flow rate and a concentrationfor a pollutant that exceeds a permitted discharge standard for thepollutant is split into a first wastewater fraction having a first flowrate and a second wastewater fraction having a second flow rate. Whenthe wastewater flow rate is no greater than a flow rate breakpoint,which is equal to the sum of the bioreactor target flow rate and theminimum flow rate, the second flow rate is approximately the minimumflow rate and the first flow rate comprises the wastewater flow rateminus the second flow rate. When the wastewater flow rate is greaterthan the flow rate breakpoint, the first flow rate is approximately thebioreactor target flow rate and the second flow rate comprises thewastewater flow rate minus the first flow rate. The first wastewaterfraction is delivered to the membrane bioreactor and an effluent fromthe membrane bioreactor is produced. This bioreactor effluent has aconcentration for the pollutant that is less than the permitteddischarge standard for the pollutant. The second wastewater fraction isdelivered to a biological wastewater treatment system that has a secondsolids retention time that is no greater than, and is desirably lessthan, the first solids retention time.

A method of treating wastewater in accordance with a further embodimentincludes delivering a first wastewater stream to a single-stagebiological treatment system containing a membrane bioreactor to producea first effluent. The first wastewater stream includes a biochemicaloxygen demand (BOD) concentration that is greater than the permitted BODdischarge standard and the first effluent has a BOD concentration thatis no greater than the permitted BOD discharge standard. A secondwastewater stream containing a wastewater pollutant concentration isdelivered to a biological wastewater treatment system to produce asecond effluent, which has an effluent pollutant concentration less thanthe wastewater pollutant. A portion of a waste activated sludge from thesingle-stage biological treatment system is delivered to the biologicalwastewater treatment system. This waste activated sludge may include atleast about 35 weight percent heterotrophs, no more than about 8 weightpercent, e.g., 3 weight percent or less, of autotrophs. Optionally, thewaste activated sludge may include at least about 10 weight percentpolyphosphate accumulating organisms.

Another alternative embodiment provides a wastewater treatment systemthat includes a first wastewater processor, a second wastewaterprocessor, an influent splitter, and a programmable controller. Thefirst wastewater processor includes a first inlet, a first outlet, and amembrane bioreactor in a first wastewater path from the first inlet tothe first outlet. The membrane bioreactor has a first solids retentiontime and a load capacity. The second wastewater processor includes asecond inlet, a second outlet, and a biological wastewater treatmentsystem in a second wastewater path from the second inlet to the secondoutlet. The biological wastewater treatment system has a second solidsretention time that is shorter than the first solids retention time. Theinfluent splitter is configured to deliver a first fraction of theinfluent to the first inlet and to deliver a second fraction of theinfluent to the second inlet. The programmable controller is operativelyconnected to the influent splitter to control flow rate parameters ofthe first and second wastewater fractions such that: when a load of thesystem influent is no greater than the membrane bioreactor loadcapacity, the first fraction has a load at least about 35% of the loadof the influent; and when the load of the influent is greater than themembrance bioreactor load capacity, the first fraction has a load thatis no less than about 50% of the membrane bioreactor load capacity.

For ease of understanding, the following discussion is separated intotwo areas of emphasis. The first section describes wastewater treatmentsystems in accordance with certain embodiments of the invention. Thesecond section outlines methods of treating wastewater in accordancewith other embodiments of the invention.

B. Wastewater Treatment Systems

FIGS. 1 and 2 schematically illustrate wastewater treatment systems 10and 100, respectively, in accordance with embodiments of the invention.The system 10 of FIG. 1 generally includes a pair of parallel biologicalsystems for treating wastewater. Wastewater is delivered to an influentsplitter 20 of the system 10 that divides an influent stream I from aninlet conduit 22 into two separate influent streams I_(a) and I_(b). Theinfluent stream I and the two separate influent streams I_(a) and I_(b)may contain one or more pollutants in amounts that exceed the permitteddischarge standard for the specific pollutant.

Different pollutants will have different permitted discharge standards,which typically are established by government agencies or regulatoryauthorities and which may be based on concentration, on load, or on bothconcentration and load. A concentration-based standard is usuallyexpressed as mass of pollutant per volume of water; a load-basedstandard is usually expressed as mass of pollutant per period of time.For example, a concentration-based permitted discharge standard may beexpressed as 2 mg P/L for phosphorous, as 10 mg N/L for total nitrogen,and as 7 mg N/L for ammonia. When the permitted discharge standard isbased on load, it may be expressed, e.g., as 200 lbs P/day forphosphorous, as 1,000 lbs N/day for total nitrogen, and as 700 lbs N/dayfor ammonia. When the permitted discharge standard is a combination ofboth load and concentration, the ammonia discharge standard might beexpressed as 700 lbs N/day for ammonia with a daily average maximumallowable concentration of 2 mg N/L for ammonia to ensure that no singleday has too high of an ammonia concentration.

Knowing the flow rate of wastewater being discharged from a plant allowsconversion of a permitted discharge standard based on load to a standardbased on concentration and vice versa. For example, if the permitteddischarge standard for ammonia based on load was 700 lbs N/day and theflow rate of the wastewater being discharged was 10 million gallons perday (MGD), then the target discharge concentration of ammonia would be8.4 mg N/L on average. Note that the load-based discharge standard willnot vary with variations in the flow rate of wastewater discharged froma plant, but a concentration-based discharge will. For example, if thepermitted ammonia discharge standard is 700 lbs N/day, which is aload-based standard, and the flow rate of the wastewater is 10 milliongallons per day (MGD), then the target effluent ammonia concentrationwould be 8.4 mg N/L (on average); if, however, the flow rate increasedto 30 MGD flow, then the permitted ammonia discharge standard based onload, which was the original standard, would remain at 700 lbs N/day butthe target ammonia discharge concentration would drop to 2.8 mg N/L.

The first influent stream I_(a) is delivered to a first wastewaterprocessor 28 via a first delivery conduit 24 a. The second influentstream I_(b) is delivered to the second wastewater processor 58 via asecond influent conduit 24 b. As discussed below, the influent splitter20 can be selectively controlled by a programmable controller 90 or anoperator to control the relative flow rates or pollutant loads of thefirst and second influent streams I_(a) and I_(b).

The first wastewater processor in FIG. 1, which in some embodiments is asingle-stage biological system, includes a membrane bioreactor 30. Themembrane bioreactor 30 may have a pollutant load capacity, a flow ratecapacity, or a pollutant load capacity and a flow rate capacity. In someembodiments, the membrane bioreactor 30 has a pollutant load capacitythat equals or exceeds the majority of anticipated pollutant loadconditions, but it has a flow rate capacity that cannot adequatelyhandle anticipated high flow events that may occur, e.g., due toprecipitation, seasonal conditions, or diurnal flow variations. Membranebioreactors generally include a holding tank 32 and a filter 34 thatincludes one or more membranes (not separately illustrated). Suitablemembrane bioreactors are generally known in the art and need not bedetailed here. PCT International Publication No. WO 00/37369 describesone membrane bioreactor design, though, and membrane filters arecommercially available from a variety of sources, including ZenonEnvironmental Inc. of Oakville, Ontario, Canada and US FilterCorporation of Warrendale, Pa., USA.

The concentration and type of microorganisms used in the membranebioreactor 30 may vary depending on the nature of the first influentstream I_(a). For example, the membrane bioreactor 30 may include one ormore of the following microorganisms: “nitrifiers” or autotrophicorganisms, which oxidize ammonia; heterotrophic organisms, whichprimarily oxidize organic material and reduce nitrates to nitrogen; andpolyphosphate accumulating organisms (PAOs). The membrane bioreactor 30of FIG. 1 may be a single-stage bioreactor, which employs one commonbiomass to reduce both ammonia and BOD content. An additives supply 36may be used to deliver any microorganisms or chemicals necessary toimprove or optimize the performance of the membrane bioreactor 30.

In one embodiment, the membrane bioreactor 30 also includes a populationof polyphosphate accumulating organisms (PAOs) and an anaerobic zone ofsufficient size to promote effective growth of the PAOs. Conventionalenhanced biological phosphorous removal employs PAOs to remove solublephosphates found in the influent I_(a). An alternative embodiment alsoemploys PAOs to remove phosphates, but does so without employing ananaerobic zone. Conventional removal of part of the membrane retentateto maintain optimal performance of the membrane bioreactor 30 results inthe net removal of phosphorous from the influent wastewater.

The fluids passing through the filter 34 define a membrane bioreactoreffluent E_(a). This effluent E_(a) may be delivered to an effluentsplitter 40 via an effluent conduit 38. The effluent splitter 40 isadapted to control the relative proportions of a first system effluentE₁ and a diverted effluent E_(c). In many embodiments, the effluentE_(a), the first system effluent E₁, and the diverted effluent E_(c) arehigh quality effluents, i.e., effluents that contain one or morepollutant concentrations less than the permitted discharge standard forthe specific pollutant. The first system effluent E₁ may be dischargedto the environment or delivered for reuse via a discharge conduit 45.The diverted effluent E_(c) may be delivered to an effluent mixingstation 70 for mixture with a biological treatment effluent E_(b),discussed below.

As noted above, the second influent stream I_(b) is delivered to asecond wastewater processor 58, which may comprise a biologicalwastewater treatment system such as a conventional activated sludgesystem 60. A wide variety of activated sludge processes are known in theart and need not be detailed here.

In the embodiment shown in FIG. 1, the activated sludge system 60includes a basin 62, which has a series of different functional zones(illustrated schematically by dashed lines). U.S. Pat. No. 5,480,548(Daigger et al.) discloses one suitable activated sludge treatmentsystem that may be employed in the basin 62.

An additives supply 66 may supply microorganisms or chemicals needed tooptimize wastewater treatment in the basin 62. In the illustratedembodiment, an oxygen supply 65, which may be a conventional bubbler,delivers an oxygen-containing gas (e.g., air) to one or more zones ofthe basin 62. This enables control of the oxygen level at one or morestages of the wastewater treatment process.

The activated sludge system 60 may also include a clarifier 64. As knownin the art, a clarifier may simply comprise a holding tank in whichmixed liquor suspended solids are allowed to settle out of suspension.Any other clarifying system known in the art, such as a chemicalclarifier, may be used instead.

The activated sludge system 60 receives the second wastewater influentstream I_(b), treats the wastewater, and delivers a biological treatmentsystem effluent E_(b), to a second effluent conduit 68. The secondeffluent conduit 68 may discharge the effluent E_(b) to the environment.In FIG. 1, therefore, the conduit 68 directs the biological treatmentsystem effluent E_(b) to the effluent mixing station 70 where thebiological treatment system effluent E_(b) may be mixed with thediverted effluent E_(c) from the effluent splitter 40. The quality ofthe diverted effluent E_(c) is typically higher than that of thebiological treatment system effluent E_(b). In some embodiments, thebiological treatment system effluent E_(b) will contain one or morepollutant concentrations that exceed the permitted concentrationdischarge standard for the specific pollutant, but the membranebioreactor effluent E_(a) may have concentrations of those pollutantsthat are less than the permitted concentration discharge standard.Mixing effluent streams E_(b) and E_(c) yields a blended effluent E₂. Bycontrolling the effluent splitter 40, the proportions of the twoincoming effluent streams E_(b) and E_(c) can be selected to produce ablended effluent E₂ that has pollutant concentrations no greater thanthe pertinent permitted concentration discharge standards. This blendedeffluent E₂ may be delivered to the environment or for reuse via outletconduit 75.

In one embodiment, the effluent mixing station 70 comprises a holdingtank, for example, a lagoon, where the two effluent streams arepassively allowed to mix. In other embodiments, the effluent mixingstation 70 may utilize mixers or other mechanical systems to activelymix the two effluent streams prior to discharge.

FIG. 2 schematically illustrates a wastewater treatment system 100 inaccordance with a further embodiment of the invention. Many elements ofthis system 100 may be essentially the same as those shown in FIG. 1 andthe same reference numbers are used in FIGS. 1 and 2 to identify likeelements.

The system 100 of FIG. 2 employs a portion of the retentate of thefilter 34 of the membrane bioreactor 30 to supply part or all of thewaste activated sludge (WAS) used in the activated sludge system 60. Asknown in the art, WAS may be removed from the filter 34 at variablerates to sustain acceptable performance of the membrane bioreactor 30.The removed WAS may be received by a WAS splitter 50, which divides theflow of WAS between a first sludge conduit 54 a and a second sludgeconduit 54 b. The first sludge conduit 54 a delivers a controlledportion (which may be selectively varied between about 0% and about100%) of the WAS from the membrane bioreactor 30 to the secondwastewater processor, typically to the first zone of the basin 62. Thesecond sludge conduit 54 may deliver the remainder of the WAS to a WASdisposal 52.

The WAS from the membrane bioreactor 30 will include both autotrophs andheterotrophs, permitting the activated sludge system 60 of FIG. 2 toeffectively oxidize organic material and reduce nitrates and nitrifyammonia. In one embodiment, the biological population of the WASincludes about 15-50 weight percent (wt. %), desirably at least about35%, oxidizing agents (e.g., heterotrophs) and no more than about 8 wt %(e.g., about 2-8 wt. %), preferably no more than about 3 wt. %,nitrifying agents (i.e., autotrophs).

In particular, the volume of WAS delivered to the basin 62 may besufficient to provide effective nitrification even under conditions thatdo not ordinarily promote effective nitrification, such as short solidsretention times. By way of example, the WAS delivered to the basin maybe selected to reduce ammonia concentration to within 25% of thepermitted ammonia discharge standard As noted above, one embodiment ofthe invention includes PAOs in the membrane bioreactor 30 to reduce thephosphate content of the membrane bioreactor effluent E_(a). In such anembodiment, the WAS transferred to the activated sludge system 60 inFIG. 2 will also include PAOs, which can reduce the phosphate content ofthe biological treatment effluent E_(b). The weight percentage of PAOsin the WAS and the selected transfer rate of the WAS may be effective toreduce the phosphorous concentration of the biological treatmenteffluent E_(b) to within 25% of, and desirably no greater than, thepermitted phosphorous discharge standard. In select embodiments, the WASincludes at least about 10 wt. % PAOs.

The programmable controller 90 may be used to control operation of thewastewater treatment system 10 in accordance with one or more of theembodiments outlined below. The programmable controller 90 may comprisea programmable computer, for example, a personal computer or anASIC-based system. The programmable controller 90 is operatively coupledto the influent splitter 20, the effluent splitter 40, and, in theembodiment of FIG. 2, the WAS splitter 50 to control the relative flowrates of the fluid streams delivered by each of these splitters. Theprogrammable controller 90 may also be coupled to other elements of thesystem 10, which are not shown in FIG. 1 or FIG. 2, to control otheraspects of the system's operation, for example, the oxygen level in themembrane bioreactor 30 and/or the activated sludge system 60.

The wastewater treatment system 10, therefore, may include at least onesingle-stage biological wastewater processor 28 arranged in parallelwith at least one other biological wastewater processor 58 such as anactivated sludge system 60. The single-stage biological wastewaterprocessor 28 includes a membrane bioreactor 30, which can treatwastewater quite effectively to deliver high-quality effluent E_(a), butit may have a relatively long solids retention time. In someembodiments, the membrane bioreactor 30 is preferred for the treatmentof a significant portion of the influent stream I. In contrast, theactivated sludge system 60 may be less effective at treating wastewater,particularly with respect to nitrogen and phosphorous removal, but itmay have a substantially shorter solids retention time. The activatedsludge system 60 can be effective for the treatment of occasional excessflow rates or pollution loads that cannot be adequately handled by themembrane bioreactor 30 without the addition of a significant number ofadditional membranes. In one embodiment, the solids retention time ofthe membrane bioreactor 30 is typically at least about 6-8 days, e.g.,6-30 days, and the activated sludge system 60 typically has a solidsretention time of 5 days or less, e.g., 0.1-5 days. In one embodiment,the solids retention time in the membrane bioreactor 30 is at leastabout 20 times the solids retention time of the activated sludge system60.

C. Methods of Treating Wastewater

Other embodiments of the invention provide methods for treatingwastewater. Many of these embodiments take advantage of the distinctbenefits of the two parallel biological wastewater processors, in otherwords, the high quality of the membrane bioreactor effluent E_(a) andthe high processing rate of the activated sludge system 60. For ease ofunderstanding, the methods outlined below are discussed with referenceto the wastewater treatment systems 10 and 100 of FIGS. 1 and 2. Themethods, however, are not limited to any particular system illustratedin the drawings or detailed above; any apparatus that enablesperformance of a method of the invention may be used instead.

In various embodiments, the incoming flow rate or the pollutant load ofthe system influent stream I can be measured and communicated to aprogrammable controller 90 or an operator. The influent can be separatedusing an influent splitter 20 into a first wastewater fraction I_(a)having a first flow rate or pollutant load and a second wastewaterfraction I_(b) having a second flow rate or pollutant load. The firstwastewater fraction I_(a) is delivered at approximately the first flowrate or pollutant load to a membrane bioreactor 30 for treatment, andthe second wastewater fraction I_(b) is delivered at approximately thesecond flow rate or pollutant load to an activated sludge system 60 fortreatment.

The sum of the first flow rate and the second flow rate may beapproximately equal to the incoming flow rate. In certain embodiments,though, the total of the first and second flow rates may be less thanthe incoming flow rate. For example, aspects of the invention augmentsystem 10 or 100 by adding one or more additional wastewater processors(not shown), which may include one or more biological processors and/orone or more chemical processors. In such a system, a portion of theincoming flow rate may be diverted to the one or more of the additionalwastewater processors.

In some embodiments, the first and second flow rate can be selectedbased on the incoming flow rate of the system influent stream I and aflow rate capacity of the membrane bioreactor 30. In one implementation,the flow rate of the first wastewater fraction I_(a) may be maintainedin an operating range that is at or near the bioreactor flow ratecapacity. Maintaining relatively constant conditions can enhance theefficiency of the membrane bioreactor 30. When the incoming flow rate ofthe system influent stream I is less than the flow rate capacity of themembrane bioreactor 30, the first flow rate is selected to be no lessthan a significant portion, preferably at least a majority, of theincoming flow rate of the system influent stream I. For example, thefirst flow rate may be at least about 65%, e.g., about 80% or greater,of the system influent flow rate.

When the incoming flow rate of the system influent stream I is greaterthan the flow rate capacity of the membrane bioreactor 30, the firstflow rate is selected to be no less than a substantial majority of theflow rate capacity of the membrane bioreactor 30. For example, the firstflow rate may comprise at least about 75%, e.g., 80% or more andoptimally at least about 90%, of the bioreactor flow rate capacity.

At times, the second flow rate delivered to the activated sludge system60 could be zero. The activated sludge system 60 may require a minimumflow rate and/or pollution load to sustain biological activity, though.In such an embodiment, the second flow rate may be maintained at orabove this minimum sludge system flow rate and/or pollution load, evenif this requires that less of the system influent I than otherwisedesirable is directed to the membrane bioreactor 30.

In one implementation, the wastewater treatment system (e.g., system 10)may have an influent flow rate breakpoint that is the sum of the minimumsludge system flow rate and a bioreactor target flow rate. Thebioreactor target flow rate may be selected to optimize quality andthroughput of the membrane bioreactor 30; in many applications, thebioreactor target flow rate may be the bioreactor flow rate capacity. Ifthe flow rate of the system influent stream I is at or below thisbreakpoint, the minimum sludge system flow rate may be delivered to theactivated sludge system 60 and the balance of the system influent flowrate may be delivered to the membrane bioreactor 30. If the flow rate ofthe system influent stream I is greater than the influent flow ratebreakpoint, the flow rate of the first wastewater fraction I_(a)delivered to the membrane bioreactor 30 is a substantial majority (e.g.,75%, 80%, or even 90% or greater) of the bioreactor flow rate capacityand the balance of the system influent flow rate may be delivered to theactivated sludge system 60.

In some embodiments, the first flow rate of influent delivered to themembrane bioreactor 30 is kept substantially constant for a selectperiod of time, e.g., 12 hours or more, while the second flow rate ofinfluent delivered to the activated sludge system 60 can vary during theselect period of time. The variations in the second flow rate may bedirectly proportional to the variations in the incoming flow rate of thesystem influent stream I. If the activated sludge system 60 has aminimum required flow rate for stable operation, the second flow rate isselected to be no less than the minimum flow rate.

In some embodiments, the first and second load for a pollutant can beselected based on the incoming pollutant load of the system influentstream I and the membrane bioreactor's 30 load capacity for thepollutant. When the incoming pollutant load of the system influentstream I is less than the pollutant load capacity of the membranebioreactor 30, then the first pollutant load is selected to be no lessthan a significant portion of the incoming pollutant load of the systeminfluent stream I. For example, the first pollutant load may be at leastabout 35%, preferably at 65%, of the incoming pollutant load. When theincoming pollutant load of the system influent stream I is greater thanthe pollutant load capacity of the membrane bioreactor 30, then thefirst pollutant load is selected to be no less than a substantialportion of the pollutant load capacity of the membrane bioreactor 30.For example, the first pollutant load may be at least about 50%,preferably at least 75%, of the pollutant load capacity of the membranebioreactor 30.

The sum of the first pollutant load and the second pollutant load may beapproximately equal to the incoming pollutant load. In certainembodiments, however, the total of the first and second pollutant loadmay be less than the incoming pollutant load. As noted above, aspects ofthe invention augment system 10 or 100 by adding one or more additionalwastewater processors. In such a system, a portion of the incomingpollutant load may be diverted to one or more additional processors.

Once the first and second flow rate or pollutant load are selected, theprogrammable controller 90 or the operator can adjust the influentsplitter 20 to obtain the desired first flow rate or pollutant load forthe first wastewater fraction and second flow rate or pollutant load forthe second wastewater fraction.

The effluent E_(b) of the sludge system 60 may exceed the maximumconcentration of one or more pollutants, e.g., total nitrogen, ammonia,nitrate, nitrite, or phosphorous, acceptable for discharge from thesystem 10. In one embodiment, the bioreactor effluent E_(a) typicallyhas one or more pollutant concentrations, for example, total nitrogen,ammonia, nitrate, nitrite, or phosphorous levels, below that maximum. Bymixing the lower-quality sludge system effluent E_(b) with thehigher-quality bioreactor effluent E_(a) in suitable proportions, thesecond effluent E₂ of the system 10 can meet the discharge requirementsand be discharged from the system 10. In one implementation, thecontroller 90 may control the effluent splitter 40 to control the amountof the bioreactor effluent E_(a) delivered to the effluent mixingstation 70 to ensure that the second effluent E₂ meets the dischargerequirements. If so desired, the controller may deliver to the effluentmixing station 70 no more than the minimum necessary to produce a secondeffluent E₂ that just meets the discharge requirements or has a qualitythat surpasses the discharge requirements by no more than a selectedsafety margin. This will increase the volume of the high-quality firsteffluent E₁, which may have more beneficial reuse options than thesecond effluent E₂.

In further embodiments of the invention, the composition of the sludgesystem effluent E_(b) may be adjusted to further improve the quality ofthe total system effluent (E₁+E₂). In one embodiment, the controller 90adjusts conditions in the activated sludge system 60 to affect thecomposition of the resultant effluent E_(b). In particular, thecontroller 90 may receive information regarding the phosphorous ornitrogen content of the sludge system effluent E_(b) and adjust one ormore operating parameters of the sludge system 60. In one embodiment,the controller 90 may control the flow rate of WAS to the activatedsludge system 60 by controlling the WAS splitter 50. If the phosphorousor nitrogen content is too high but the mixed liquor suspended solidsare at an acceptable level, more WAS may be delivered to the activatedsludge system 60; if the mixed liquor suspended solids content gets toohigh, WAS may be delivered more slowly.

In another embodiment, the controller 90 controls the rate at whichoxygen is delivered to the basin 62 by the oxygen supply 65 to affectnitrification. As is known in the art, effective nitrification usuallyrequires the water in the basin 62 to have a minimum oxygen content. Thecontroller 90 may limit the oxygen level in the water in the basin 62 toa level normally incapable of supporting nitrification; this can reduceoperating costs in some embodiments. The activated sludge system 60 canstill achieve acceptable nitrification of the influent I_(b) byincreasing delivery rate of WAS, which will increase the initialpopulation of nitrifiers. The activated sludge system 60 may be unableto support a sufficient population of nitrifiers on its own, but theaddition of more nitrifiers from the WAS will promote more nitrificationthan the activated sludge system would otherwise support.

The above-detailed embodiments and examples are intended to beillustrative, not exhaustive, and those skilled in the art willrecognize that various equivalent modifications are possible within thescope of the invention. For example, steps presented in a given ordermay be performed in a different order in alternative embodiments. Thevarious embodiments described herein can be combined to provide furtherembodiments.

In general, the terms used in the following claims should not beconstrued to limit the invention to the specific embodiments disclosedin the specification unless the preceding description explicitly definessuch terms. The inventors reserve the right to add additional claimsafter filing the application to pursue additional claim forms for otheraspects of the invention.

1. A method of treating wastewater comprising: splitting a wastewaterhaving a concentration of a pollutant into a first wastewater fractionand a second wastewater fraction; delivering to a membrane bioreactorthe first wastewater fraction having a first concentration of thepollutant that exceeds the permitted discharge standard for thepollutant and producing an effluent from the membrane bioreactor havinga concentration of the pollutant less than the permitted dischargestandard for the pollutant; delivering to a biological wastewatertreatment system the second wastewater fraction having a secondconcentration of the pollutant that exceeds the permitted dischargestandard for the pollutant and producing an effluent from the biologicalwastewater treatment system having a concentration of the pollutantgreater than the permitted discharge standard; and combining at least aportion of the effluent from the membrane bioreactor with at least aportion of the effluent from the biological wastewater treatment systemto create a blended effluent having a concentration of the pollutant nogreater than the permitted discharge standard.
 2. The method of claim 1wherein delivering the first wastewater fraction and delivering thesecond wastewater fraction comprise varying a flow rate of the secondwastewater fraction while maintaining a flow rate of the firstwastewater fraction.
 3. The method of claim 1 wherein the pollutant isat least one pollutant selected from a group consisting of biochemicaloxygen demand, chemical oxygen demand, total suspended solids, ammonia,nitrate, nitrite, total nitrogen, and phosphorous.
 4. The method ofclaim 1 wherein the membrane bioreactor generates a waste activatedsludge, the method further comprising delivering a portion of the wasteactivated sludge to the biological wastewater treatment system.
 5. Themethod of claim 4 wherein the waste activated sludge comprises at leastabout 35 weight percent heterotrophs and no more than about 3 weightpercent autotrophs.
 6. The method of claim 4 wherein the waste activatedsludge comprises at least about 10 weight percentpolyphosphate-accumulating organisms.
 7. A method of treating wastewatercomprising: splitting a wastewater flow having a load for a pollutantthat exceeds a permitted discharge standard for the pollutant into afirst wastewater fraction having a first load for the pollutant and asecond wastewater fraction having a second load for the pollutant,wherein when the load is no greater than a load capacity of a membranebioreactor that has a first solids retention time, the first load is atleast about 35% of the load, and when the load is greater than the loadcapacity of the membrane bioreactor, the first load is no less thanabout 50% of the load capacity of the membrane bioreactor; deliveringthe first wastewater fraction containing approximately the first load tothe membrane bioreactor and producing an effluent from the membranebioreactor having a load for the pollutant less than the permitteddischarge standard for the pollutant; and delivering the secondwastewater fraction containing approximately the second load to abiological wastewater treatment system having a second solids retentiontime, the second solids retention time being no greater than the firstsolids retention time.
 8. The method of claim 7 wherein the first loadis no less than about 75% of the load capacity of the membranebioreactor when the load is greater than the load capacity of themembrane bioreactor.
 9. The method of claim 7 wherein an effluent fromthe biological wastewater treatment system has a concentration for thepollutant greater than the permitted discharge standard for thepollutant.
 10. The method of claim 9 further comprising combining aportion of the effluent from the membrane bioreactor with a portion ofthe effluent from the biological wastewater treatment system to create ablended effluent having a concentration for the pollutant no greaterthan the permitted discharge standard for the pollutant.
 11. The methodof claim 10 wherein the pollutant is at least one pollutant selectedfrom a group consisting of biochemical oxygen demand, chemical oxygendemand, total suspended solids, ammonia, nitrate, nitrite, totalnitrogen, and phosphorous.
 12. The method of claim 7 wherein deliveringthe first wastewater fraction and delivering the second wastewaterfraction comprise varying a flow rate of the second wastewater fractionwhile maintaining a flow rate of the first wastewater fraction.
 13. Themethod of claim 7 wherein the membrane bioreactor generates a wasteactivated sludge, the method further comprising delivering a portion ofthe waste activated sludge to the biological wastewater treatmentsystem.
 14. The method of claim 13 wherein the waste activated sludgecomprises at least about 35 weight percent heterotrophs and no more thanabout 3 weight percent autotrophs.
 15. The method of claim 13 whereinthe waste activated sludge comprises at least about 10 weight percentpolyphosphate-accumulating organisms.
 16. A method of treatingwastewater comprising: splitting a wastewater flow having a flow rateand a concentration for a pollutant that exceeds a permitted dischargestandard for the pollutant into a first wastewater fraction having aflow rate and a second wastewater fraction having a flow rate, whereinwhen the flow rate of the wastewater is no greater than a flow ratecapacity of a membrane bioreactor that has a first solids retentiontime, the flow rate of the first wastewater fraction is at least about50% of the flow rate of the wastewater, and when the flow rate of thewastewater is greater than the flow rate capacity of the membranebioreactor, the flow rate of the first wastewater fraction is no lessthan about 50% of the flow rate capacity of the membrane bioreactor;delivering the first wastewater fraction to the membrane bioreactor andproducing an effluent from the membrane bioreactor having aconcentration for the pollutant less than the permitted dischargestandard for the pollutant; and delivering the second wastewaterfraction to a biological wastewater treatment system having a secondsolids retention time, the second solids retention time being no greaterthan the first solids retention time.
 17. The method of claim 16 whereinthe flow rate of the first wastewater fraction is no less than about 75%of the flow rate capacity of the membrane bioreactor when the flow rateof the wastewater is greater than the flow rate capacity of the membranebioreactor.
 18. The method of claim 16 wherein an effluent from thebiological wastewater treatment system has a concentration for thepollutant greater than the permitted discharge standard for thepollutant.
 19. The method of claim 18 further comprising combining aportion of the effluent from the membrane bioreactor with a portion ofthe effluent from the biological wastewater treatment system to create ablended effluent having a concentration for the pollutant no greaterthan the permitted discharge standard for the pollutant.
 20. The methodof claim 19 wherein the pollutant is at least one pollutant selectedfrom a group consisting of biochemical oxygen demand, chemical oxygendemand, total suspended solids, ammonia, nitrate, nitrite, nitrogen, andphosphorous.
 21. The method of claim 16 further comprising varying theflow rate of the second wastewater fraction.
 22. The method of claim 16wherein the membrane bioreactor has a load treatment capacity for thepollutant that is greater than a maximum load of the pollutant presentin the wastewater during a majority of time.
 23. The method of claim 16wherein the flow rate of the second wastewater fraction is no less thana minimum flow rate required to maintain stable operation of thebiological wastewater treatment system.
 24. The method of claim 16wherein the membrane bioreactor generates a waste activated sludge, themethod further comprising delivering a portion of the waste activatedsludge to the biological wastewater treatment system.
 25. The method ofclaim 24 wherein the waste activated sludge comprises at least about 35weight percent heterotrophs and no more than about 3 weight percentautotrophs.
 26. The method of claim 24 wherein the waste activatedsludge comprises at least about 10 weight percentpolyphosphate-accumulating organisms.
 27. A method of treatingwastewater with a system including a membrane bioreactor, which has abioreactor target flow rate and a first solids retention time, and abiological wastewater treatment system having a minimum flow rate,comprising: splitting a wastewater flow having a wastewater flow rateand a concentration for a pollutant that exceeds a permitted dischargestandard for the pollutant into a first wastewater fraction having afirst flow rate and a second wastewater fraction having a second flowrate, wherein when the wastewater flow rate is no greater than a flowrate breakpoint equal to the sum of the bioreactor target flow rate andthe minimum flow rate, the second flow rate is at least the minimum flowrate and the first flow rate comprises the wastewater flow rate minusthe second flow rate; and when the wastewater flow rate is greater thanthe flow rate breakpoint, the first flow rate is at least the bioreactortarget flow rate and the second flow rate comprises the wastewater flowrate minus the first flow rate; delivering the first wastewater fractionto the membrane bioreactor and producing an effluent from the membranebioreactor having a concentration for the pollutant less than thepermitted discharge standard for the pollutant; and delivering thesecond wastewater fraction to a biological wastewater treatment systemhaving a second solids retention time, the second solids retention timebeing no greater than the first solids retention time.
 28. The method ofclaim 27 wherein the bioreactor target flow rate comprises a flow ratecapacity of the membrane bioreactor.
 29. A method of treating wastewatercomprising: delivering a first wastewater stream to a single-stagebiological treatment system including a membrane bioreactor to produce afirst effluent, wherein the first wastewater stream includes abiochemical oxygen demand (BOD) concentration that is greater than apermitted BOD discharge standard and the first effluent has a BODconcentration that is no greater than the permitted BOD dischargestandard; delivering a second wastewater stream containing a wastewaterpollutant concentration to a biological wastewater treatment system toproduce a second effluent having an effluent pollutant concentrationless than the wastewater pollutant concentration; and delivering aportion of a waste activated sludge from the single-stage membranebioreactor to the biological wastewater treatment system.
 30. The methodof claim 29 wherein the waste activated sludge comprises at least about35 weight percent heterotrophs and no more than about 3 weight percentautotrophs.
 31. The method of claim 29 wherein the waste activatedsludge comprises at least about 10 weight percentpolyphosphate-accumulating organisms.
 32. The method of claim 29 whereinthe effluent pollutant concentration of the second effluent is greaterthan a permitted discharge standard for the pollutant.
 33. The method ofclaim 32 wherein the pollutant is at least one pollutant selected from agroup consisting of biochemical oxygen demand, chemical oxygen demand,total suspended solids, ammonia, nitrate, nitrite, total nitrogen, andphosphorous.
 34. The method of claim 32 wherein the pollutant is totalnitrogen.
 35. The method of claim 34 further comprising combining aportion of the first effluent with a portion of the second effluent tocreate a blended effluent having a total nitrogen concentration lessthan a permitted total nitrogen discharge standard.
 36. The method ofclaim 32 wherein the pollutant is phosphorous.
 37. The method of claim36 further comprising combining a portion of the first effluent with aportion of the second effluent to create a blended effluent having aphosphorous concentration less than a permitted phosphorous dischargestandard.
 38. The method of claim 37 wherein the waste activated sludgecomprises at least about 10 weight percent polyphosphate-accumulatingorganisms.
 39. The method of claim 32 wherein the pollutant is ammonia.40. The method of claim 39 further comprising combining a portion of thefirst effluent with a portion of the second effluent to create a blendedeffluent having a ammonia concentration less than a permitted ammoniadischarge standard.